Investigators have developed a variety of technical approaches to the generation of coordinate pair signals from electrographic devices. Industrial requirements for these devices are increasing concommitantly with the evolution of computer graphics, computer-aided design, and computer-aided manufacturing systems. Thus, a considerable degree of accuracy in pinpointing physical conditions upon the surfaces of the digitizers is required for many applications.
The operation of a digitizer or graphics tablet generally involves the same manual procedures as are employed in conventional graphics design, generally, a stylus representing a writing instrument being drawn across or selectively positioned upon the surface of the digitizer. In turn, the electrographic device responds to the position of the stylus to generate analog paired coordinate signals which are digitized and conveyed to a host computer facility.
A variety of design approaches have been employed in the fabrication of digitizer structures. The more recent and improved approaches to such designs have been to resort to a capacitive coupling of the stylus with the position responsive surface to generate analog paired coordinate signals. Such capacitive coupling can be carried out either with a grid layer which is formed of spaced linear arrays of conductors or through resort to the use of an electrically resistive material layer or coating.
An immediately apparent advantage of developing position responsive surfaces or digitizers having writing surfaces formed of a continuous resistive material resides in the inherent simplicity of merely providing a resistive surface upon a supportive substrate such as glass or plastic. Further, unlike conventionally encountered grid structures, the resistive coatings as well as their supportive substrates may be transparent to considerably broaden the industrial applications for the devices. For example, the digitizers may be placed over graphics or photographic materials for the purpose of tracing various profiles.
A variety of technical problems have been encountered in the development of resistive coating type digitizer devices, one of which concerns the non-uniform nature of the coordinate readouts achieved with the surfaces. Generally, precise one-to-one correspondence or linearity is required between the actual stylus position and the resultant coordinate signals. Because the resistive coatings cannot be practically developed without local resistance (thickness) variations, the nonlinear aspect of the otherwise promising approach has required a considerable amount of investigation and development. Exemplary of such development is the border treatment or switching technique of Turner in U.S. Pat. No. 3,699,439 entitled "Electrical Probe-Position Responsive Apparatus and Method", issued Oct. 17, 1972, and assigned in common herewith. This approach uses a direct current form of input to the resistive surface from a hand held stylus, the tip of which is physically applied to the resistive surface. Schlosser et al. describes still another improvement wherein an AC input signal is utilized in conjunction with the devices and signal treatment of the resulting coordinate pair output signals are considerably improved. See U.S. Pat. No. 4,456,787 entitled "Electrographic System and Method", issued June 26, 1984, also assigned in common herewith. Position responsive performance of the resistive layer devices further has been improved by a voltage wave form zero crossing approach and an arrangement wherein AC signals are applied to the resistive layer itself to be detected by a stylus or tracer as described in U.S. Pat. No. 4,055,726 by Turner et al. entitled "Electrical Position Resolving By Zero Crossing Relay", issued Oct. 25, 1977 and also assigned in common herewith. Substantially improved accuracies for the resistive surface-type digitizer devices have been achieved through a correction procedure wherein memory retained correction data are employed with the digitizer such that any given pair of coordinate signals are corrected in accordance with data collected with respect to each digitizer resistive surface during the manufacture of the digitizers themselves. With such an arrangement, the speed of correction is made practical and the accuracy of the device is significantly improved. The correction table improvements for these surfaces is described, for example, in Application for United States Patent, Ser. No. 06/664,980 Oct. 26, 1984 by Nakamura et al. and assigned in common herewith now as well as in Application for United States Patent, Ser. No. 06/742,733, entitled "Electrographic System and Method", filed June 7, 1985 by Nakamura et al. and assigned in common herewith, now U.S. Pat. No. 4650,926.
Capacitive coupling using a stylus has been employed with grid layers which are formed as adjacent but spaced-apart arrays of elongate thin conductors. For example, these conductors may be provided as lines of silver ink deposited in orthogonally disposed relationship upon opposite faces of a sheet of insulative material such as Mylar. A resistance network is employed with each of the conductor arrays such that a predetermined resistance is coupled between each conductor from first to last. A technique considerably improving this form of grid array digitizer wherein the grid arrays themselves are excited by an AC signal for data collection from a stylus is described in an application for United States Patent by Kable entitled "Position Responsive Apparatus, System and Method Having Electrographic Application", Ser. No. 791,324 filed Oct. 25, 1985 and assigned in common herewith, Abandoned.
The stylus structures which are utilized with digitizer tablets also must be designed to avoid error associated with their capacitive coupling to a surface. This is particularly true in the preferred case of digitizer structures wherein the surfaces thereof themselves are excited by an AC current. Generally, the hand and body of the user of the stylus carries part of this AC signal to generate error induced into the stylus from the hand itself. Further, early stylus structures have been found to exhibit error in position data acquisition due to the common technique by which the user holds the stylus. In this regard, the comfortable position for holding the stylus is that which normally would be employed in holding a pencil or other conventional writing device, i.e. at an angle with respect to the plane of the digitizer surface. When such angularity of retention is employed by the user, early devices exhibited a variation of capacitive coupling geometry such that the resultant coordinate pair position data were inaccurate. Further, it is necessary for the operator to signal to the digitizer circuitry that a position wherein coordinate pair data are desired has been reached. This often has been accomplished through a switch which is actuable by the finger of the user. A more convenient approach has been to employ a cartridge technique wherein the user presses the point of the stylus into the digitizer surface to actuate the switch. This form structuring led to inaccuracies due to the length of the pickup capacitive coupling rod which was exposed to the surface varying somewhat for each position desired. To avoid scratching the delicate digitizer surfaces, it has been found desirable to cover the tip of the stylus with some form of plastic material. However, with the provision of such covering or insertion of the pickup rod or conductor within the plastic tip or the like, static electricity has tended to build within the stylus device itself. Finally, it has been found desirable to provide some technique by which the user or operator can be appraised of the acceptance by a host computer of the location of coordinate pair data upon the actuation of some stylus mounted switching device.